Allow trimmed whitespace to generate empty fragments.

BREAKING: Output may now contain more fragments with zero glyphs,
representing runs of whitespace that was trimmed away. These runs may
also start a box at the end of a line, instead of the beginning of the
next line.

BREAKING: Trimmed whitespace now affects line height.

Internally allow splitting texts at endpoints.

This is required in order to preserve a hard line break at the end of
the input.

Fix style: white space around operators.

Mark some lists as explicitly non-empty.

Remove collapsible spaces at beginning of lines.

Refactor line breaking.

Implement trimming spaces at end of lines.

Move non-public modules into Internal namespace.

Minimise working with Data.Text.Internal.

Implement run splitting.